Hidden behind the Mona Lisa's enigmatic smile are millions of invisible dots, according to research presented this week at the Uffizi Gallery in Florence, Italy.

Revealed at the exhibition "The mind of Leonardo — The universal genius at work," which runs at the gallery until January 2007, the Mona Lisa code consists of countless of dot layers applied with a technique of micro-divided brushstrokes.

Jacques Franck, a consultant at the Armand Hammer Centre for Da Vinci Studies at the University of California, reported that the technique is somewhat similar to pointillism used by the French Neo-Impressionists in the late 19th century.

"Examples of this micro-division of tones exist since the ancient Romans. Leonardo took an existing techniques, but used it to the extreme, like nobody else," Franck told Discovery News.

Called "sfumato," from the Italian word "fumo" (meaning smoke), the painting technique produces an almost three-dimensional effect, the result of the delicate brushwork that blends light, shadow, and contours.

Da Vinci never really explained how he was able to blend shadow and light in such an imperceptible way.

The only reference to the sfumato technique appears in his notes on painting: "'light and shade should blend without lines or borders, in the manner of smoke," he wrote.

Using what he calls an "archaeological approach," Franck examined Leonardo's works in search of clues.

He took a close look at another da Vinci work, "Drapery study for a seated figure," which hangs in the Louvre, and noticed that Leonardo used minute brushstrokes to produce the 3-D effect of the draperies.

"The technique is visible, as he used tempera. On the contrary, in oil painting traces would have been masked by a delicate velatura or glaze," Franck said.

An artist himself — he has been copying Leonardo's works since he was eight — Franck backed up his micro-divisionism theory by reconstructing the Mona Lisa's eye through six steps, represented by panels on display at the Uffizi exhibition.

He started with a poplar panel, similar to the one Leonardo used to paint the Mona Lisa in 1503-1506.

Franck treated the panel with gypsum and animal glue, then painted an eye with a brush, but using black chalk. Then he applied a semi-opaque, diluted oil-based wash to soften the chalk lines and then retouched the details with microscopic brushstrokes. There had to be many layers of dots applied, until they blend together.

According to Franck, details such as the smile and eyes contain between 30 and 40 brush strokes per millimeter; essentially, Leonardo was working as a miniaturist would.

"He may have made one square millimiter a day. This means it might have taken Leonardo about 10 years to complete the Mona Lisa," Franck said.

Lisa di Antonio Maria Gherardini, the Mona Lisa, has been fascinating art lovers since her portrait was completed toward the end of the life of Leonardo, who lived from 1452 to 1519.

Attempts to solve the enigma around her smile, described by the 16th century artist and writer Giorgio Vasari as "more divine than human," have included theories that the noblewoman was happily pregnant, suffering from asthma, had facial paralysis or that the smile was the result of a compulsive gnashing of teeth.

Franck's theory is raising a debate among art historians.

"I find it absolutely untrue. Leonardo did use the sfumato, but his painting technique is also very firm. I have seen the Mona Lisa under a magnifying glass and really, I could not see any dot," Da Vinci scholar and U.K. professor of art history Martin Kemp, from the University of Oxford, told Discovery News.

Franck argues that it is scientifically impossible to distinguish the dots with a magnifying glass.

"Light penetrates each dot, producing reflection, diffraction, and diffusion. Thus, when you look at the painting, the surface appears homogeneous. No dot can be seen. Moreover, in my works the dots completely disappeared within three years. The same might have happened with the Mona Lisa," Franck said.

Indeed, X-ray images of Franck's copies are very similar to X-ray pictures of the master's works: they all show something similar to smoke.

"The layers of paint are so thin that X-ray simply goes through," Franck said.

According to the curators of the exhibition, Franck's hypothesis is "convincing."

"It is compatible with the material evidence of the Mona Lisa as analysed by the Louvre's laboratory," they wrote in the exhibition catalogue.